Low Blood Pressure Overview

Low blood pressure is a difficult clinical finding for a healthcare professional to address. While high blood pressure is known as the "silent killer," because it is associated with few acute symptoms, hypotension (hypo=low + tension=pressure) may be normal for a patient if it is without symptoms, but can be of great importance if it is associated with abnormal body function. Sometimes low is good, a goal to be achieved in keeping blood pressure under control. Sometimes low is bad because there is not enough pressure to provide blood flow to the organs of the body.

Blood pressure readings have two parts and are expressed as a ratio:

"Normal" blood pressure, for example is 120/80 (120 over 80) and measures the pressure within the arteries of the body.

Systolic pressure, the upper number, measures the pressure within the arteries when the heart is contracting (systole) to pump blood to the body.

Diastole pressure, the lower number, measures resting pressures within the arteries, when the heart is at rest.

You can think of the heart and the blood vessels (arteries and veins) as a system to pump blood, just like the oil pump in your car. Oil is pumped through rigid tubes. Pressure remains relatively constant throughout the pumping cycle unless the pump fails or there is an oil leak. Then oil pressure will fall.

The body is similar, except that the tubes have pliable walls, meaning that the space within the arteries can get bigger or smaller. If the space gets bigger, there is effectively less fluid, and pressure falls. If the space gets smaller, pressure goes up. Arteries have layers of muscles within their walls that can contract and narrow the artery, making less space inside the vessels. Alternatively, the muscles can relax and dilate the artery, making more room. These muscles are under the control of the autonomic nervous system, the body's automatic system that makes adjustments for moment-to-moment changes in the relationship of the body to the world. The autonomic nervous system has two pathways that balance each other.

The sympathetic nervous system uses adrenaline (epinephrine) to cause the muscles to contract (sympathetic tone). The nerves that help with this control are located in the sympathetic trunk, which is a group of nerves that runs alongside the spinal column. The parasympathetic system uses acetylcholine to make muscles in the blood vessel walls relax via the vagus nerve. As an example, when you stand up, the blood vessels have to narrow just a little to cause a slight increase in blood pressure, so that blood can travel uphill to the brain. Without that change, you might feel lightheaded or pass out.

Normal blood pressure depends on many factors including age and body size.

Based upon American Heart Association guidelines, any reading greater than 120/80 is considered pre-hypertension or early high blood pressure.

For low blood pressure to be a problem there needs to be a symptom associated with that low number. Readings below 120/80 may be normal depending upon the clinical situation. Many people have systolic blood pressures below 100, but some people develop symptoms with pressures that low. Symptoms of low blood pressure occur because one or more of the body's organs is not getting enough blood supply.